toxic oxygen species that require scavenging by these antioxidant molecules, and also

that the major function of the enzymes of the ascorbate-glutathione cycle was to

eliminate H202 in the chloroplast.

Paraquat-treated chloroplasts have also been shown to generate the OH'.

Initially, only the electron spin resonance signal of 02 was detected. On addition of

SOD, the signal changed to that of OH' (Dodge, 1989). Babbs et al. (1989) have

detected OH* in paraquat-treated plants at levels that were considered to be lethal. A

reaction proposed by Winterbour (1981) between the paraquat radical and H202

giving rise to the hydroxyl radical in the absence of a metal catalyst has since been

discounted (Halliwell and Gutteridge, 1989). Other highly reactive species have been

proposed to be generated in the presence of paraquat. These include 'crypto-OH'

(Youngman and Elstner, 1981; Youngman, 1984) and an iron (IV) species (Sutton et

al., 1987).

In summary, paraquat applied to illuminated plants is reduced by PSI, which

decreases the NADPH concentration and forms paraquat radicals. Under the aerobic

conditions of the chloroplast, the paraquat radicals are instantly reoxidized by 02

which generates 02-. Superoxide is dismutated either spontaneously or by SOD to

form H202. The H202 not only blocks carbon fixation but also affects the ascorbate-

glutathione pathway since there is little reducing power for the functioning of

glutathione reductase. The enhanced production of 02 and H202 and their impaired

scavenging results in the formation of OH'. The indiscriminate reactivity of OH*

radicals leads to the destruction of cell constituents. One major consequence is the

peroxidation of cell membranes, including the tonoplast. There is a loss of osmotic

control and fluctuation in pH. The rupture of the tonoplast causes the release of the

toxic contents of the vacuole contributing to the general deterioration of the cell

(Dodge, 1989).

Herbicide Resistance

With the increased occurrence of weed resistance to herbicides, reports on the

subject in the literature have become more frequent. There has been a problem of

inconsistency in the use of the terms tolerant and resistant. It was recently suggested

within a committee of the Weed Science Society of America that the term resistance

should supersede tolerance. The decision on terminology has been deferred until

other pest disciplines could be consulted on their terminology (Thill, 1993). The use

of the term resistance in this document is based on the definition by Warwick (1991).

Herbicide resistance may be defined as the condition whereby a plant
withstands the normal field dose of a herbicide, as a result of selection and
genetic response to repeated exposure to herbicides with a similar mode of
action. Susceptible plants are normally killed by recommended field doses.
(Warwick, 1991, page 95).

LeBaron and McFarland (1990) have compiled some characteristics of

herbicides that predispose weeds to develop resistance. Herbicides that target a single

site of action; those that effectively control a wide range of weed species; soil-applied

herbicides with residual activity that control germinating weeds throughout the growth

season, or are applied several times per season and; recurrent applications over

several seasons without rotating, alternating or combining with other types of

herbicides.

Although herbicide resistance has an economic cost because of a requirement

for alternative methods of control, there have also been benefits. The study of

herbicide resistant weeds has expanded our knowledge of many aspects of plant

biology and herbicide mode of action (LeBaron and McFarland, 1990). They are a

Figure 19. Production of NADPH in isolated thylakoids of paraquat-S (A) and -R (o)
biotypes as measured by absorbance at 340 nm. Closed symbols indicate NADPH
production in the presence of at 100 /M paraquat.

2.50

S1.00 -

0.50 -
0.50 0"----'-------------'----"---------'0----0-

0.00 1 1 1 I
0 3 6 9 12 15

Time (min)

Figure 20. Production of NADPH in isolated thylakoids of paraquat-S (A) and -R (o)
as measured by absorbance at 340.

The magnitude of the response in these two experiments varied because the

chloroplasts used in the second experiment were slightly aged because they had been

isolated the previous day. This tends to reduce the activity of the chloroplasts. Since

the chloroplasts were isolated from two different groups of plants, this may also

account for some of the observed variability in the response.

Oxygen Radical Generation

Similar results were obtained in both experiments in which 02- generation was

measured. The data were not combined since the experiments were not performed

over the same time period, and there were differences in the way in which the

experiments were conducted. Figure 21 illustrates the data collected from the first